The present invention relates to a series of new derivatives of the compounds known as "milbemycins", particularly of milbemycin A.sub.3, milbemycin A.sub.4 and milbemycin D.
Milbemycin D was disclosed in U.S. Pat. No. 4,346,171, where it was referred to as "compound B-41D", and milbemycins A.sub.3 and A.sub.4 were disclosed in U.S. Pat. No. 3,950,360. These compounds may be represented by the formula (I): ##STR1## in which R.sup.1 represents a methyl group, an ethyl group or an isopropyl group, these compounds being milbemycin A.sub.3, milbemycin A.sub.4 and milbemycin D, respectively. The numbering system shown on the above formula is that employed herein for all milbemycin derivatives, including the compounds of the present invention.
These milbemycin compounds may be isolated from cultures of the Streptomyces strain B-41-146, which has been deposited at the Fermentation Research Institute, Agency of Industrial Science and Technology, Ministry of International Trade and Industry, Japan, whence it is available under the accession number FERM-1438. The compounds have been found to have valuable anthelmintic and acaricidal activities.
Certain 5-(lower alkanoyl)oxy derivatives of milbemycin D are disclosed in Japanese patent application Kokai (i.e. laid open to public inspection) No. 120589/82. U.S. Pat. No. 4,201,861 discloses C-076 macrolide derivatives (similar to the milbemycins) in which there is an acyloxy group or a sugar-oxy group at the 5-position.
We have now discovered a series of derivatives of milbemycins A.sub.3, A.sub.4 and D which have demonstrated, in certain test systems, activities against specific endoparasites and ectoparasites better than the corresponding activities of their parent compounds.